1. Field of the Invention
This invention relates to a pneumatic tire having a bead core comprised of a core wire and plural sheath layers laminated therearound.
2. Description of Related Art
In general, a large stress is applied to a bead portion in a pneumatic tire used under a high speed and a heavy load, e.g. a pneumatic tire for an airplane during the running of the tire. Therefore, a bead core durable to such a large stress should be arranged in the bead portion.
As the bead core durable to the large stress, there has hitherto been known a bead core as disclosed in JP-A-53-51804 or so-called cable bead formed by helically winding plural thin diameter wires around a single thick-diameter wire to form a sheath layer and laminating a plurality of such sheath layers one upon the other. In such a bead core, each of the sheath layers is formed by winding the sheath wires at a close-packed state so as to bear the above large stress. The term "close-packed state" used herein means a state that a maximum number of sheath wires are embedded in each of the sheath layers.
However, when the pneumatic tire having such a conventional bead core is used over a long time, there is a problem that fatigue breakage is caused in the sheath wires of a second sheath layer from an outermost side to lower bead portion durability as mentioned below. That is, since each of the sheath layers is constructed by winding the sheath wires at a close-packed state and the number of sheath wires embedded at the close-packed state is necessarily a positive integer, outer surfaces of adjoining sheath wires (bare wires) are not usually closed to each other, so that a slight gap is existent between the sheath wires (a sum of all gaps in one sheath layer is more than zero but is smaller than a diameter of one sheath wire). In the vulcanization of such a pneumatic tire, therefore, rubber disposed around the bead core flows into the inside of the bead core through a slight gap between adjoining sheath wires in a radial direction. For example, in case of the pneumatic tire for airplanes, rubber covers a full periphery of a sheath wire in an outermost sheath layer and flows inward into a second sheath layer from the outermost side in the radial direction through gaps between the sheath wires of the outermost sheath layer so as to cover about 10-20% of an outward portion of each sheath wire in the second sheath layer in the radial direction. For this end, in the conventional bead core, a greater part of adjoining sheath wires in the second sheath layer from the outermost side are opposite to each other at a bare state, while all adjoining sheath wires from a third and on sheath layers are opposite to each other at a bare state. At this state, when such a pneumatic tire is run at a high speed under a heavy load, a deformation quantity produced in each sheath layer (sheath wires) becomes larger as the sheath layer becomes near to the outermost side, so that the adjoining bared sheath wires in the second sheath layer directly contact with each other to strongly cause fretting and hence a large stress is repeatedly caused in the inside of the sheath wire in the second sheath layer to finally cause fatigue breakage. Although the deformation quantity produced in the outermost sheath layer is largest among the plural sheath layers, the full periphery of each sheath wire in the outermost sheath layer is covered with rubber as mentioned above, so that the adjoining sheath wires do not directly contact with each other and hence the fatigue breakage hardly occurs in the sheath wires of the outermost sheath layer.